Cellular telephone systems have become very popular in many countries throughout the world. Unfortunately, the specific standards adopted by different countries are often different and cellular devices from one country will not be operable within another system. Accordingly, completely different handsets are necessary if a person is operating in two different countries having different standards.
As a result of this difficulty, there have been some efforts to provide a single device that is operable in different countries having different standards. A technology known as software-defined radio (SDR) provides one solution to this problem. In the SDR system, the entire band of RF or IF signal is digitized and the channel is selected using a programmable digital filter. Thus, the SDR can be reconfigured through the software to suit different standards. However, this effort has not been completely successful because it requires a high-speed A/D converter that must have not only high-speed, but also provide adequate resolution. In the ideal SDR arrangement, the RF is directly digitized in the receiver. It requires that the A/D converter have a speed in the gigahertz range, and also have a dynamic range over a 100 dB, which is equivalent to a 16-bit resolution. Among the various A/D converters, the sigma-delta A/D converter has promise in achieving the desired dynamic range. However, it is only achievable at a much lower frequency band. As the frequency increases, circuit imperfections become dominant and degrade the dynamic range of the A/D converter. At the gigahertz sampling frequency range, the highest dynamic range for the reported bandpass sigma-delta modulator is 75 dB, which only corresponds to a 12.5 bit resolution.
A sigma-delta A/D converter consists of a sigma-delta modulator and a digital filter. FIG. 1 shows a typical arrangement of a bandpass sigma-delta modulator. The key element in the modulator is the resonator 2 which provides the quantization noise shaping. That is, the resonator acts as a bandpass filter in a band around its resonance frequency. The resonator needs to operate at a high frequency and have a high Q (quality factor) value. The output of the resonator is passed to a quantizer 3 which produces a digital “1” signal if its input exceeds a threshold and a digital “0” signal if the input is less than the threshold. This digital signal of a series of 0's and 1's becomes the output of the sigma-delta modulator. These signals are also fed back to a D/A converter 4, and the resultant analog signal is applied as a second input to summation device 1 which also receives the input to the modulator. The difference between the input and the feedback of the summation device produces an input to the resonator.
FIG. 2 shows a typical output spectrum from a fourth order bandpass sigma-delta modulator where the quantization noise is shaped away from the resonance frequency, resulting in a very high dynamic range. The depth of the notch is related to the Q value of the resonator. The higher the Q value is, the deeper the notch. The noise shaping is also dependent on the order of the modulator. Higher order modulators provide better noise shaping and hence a higher dynamic range.
Typically, the resonator is made of one of three different electronic circuits, namely, a passive L-C tank, a transconductor-capacitor or a switch-capacitor. However, none of these circuits have been successful in the situation described. The first two circuits cannot achieve a high Q value due to parasitic losses and non-linearity. Typical Q values are around 10 and 40 for the integrated L-C tank with and without Q enhancement, respectively. The enhanced Q value for the transconductor-capacitor resonator can be up to 300 at a frequency of several hundred MHz. The switch-capacitor resonator is restricted by its low resonant frequency (<100 MHz) due to the slow settling behavior of the circuit. The use of these types of resonators prevents the sigma-delta modulator from achieving a high speed and high dynamic range at the same time as is required in an SDR situation. In order to achieve a workable SDR system, it is necessary to find a resonator which is usable in a sigma-delta modulator to achieve high speed and high dynamic range.